Many systems include imaging devices to sense and capture optical images that can be electronically converted to a digital representation of the image. Image sensors include an array of photosensitive devices such as photodiodes or photo-transistors fabricated on, for example, a silicon wafer. Each photosensitive device is sensitive to light in such a way that it can create an electrical charge that is proportional to the intensity of light striking the photosensitive device. The overall image captured by an image sensor includes many pixels arranged in an array such that each pixel detects the light intensity at the location of that pixel. A single pixel may include a single photosensitive device configured for detecting a broad frequency range, which may be used for gray scale images. In addition, a pixel may be defined as a single photosensitive device configured for detecting a specific color (i.e., frequency). Finally, a pixel may be defined as a group of photosensitive devices arranged near each other wherein different devices within the group are configured for detecting different colors. Thus, a full color image may be detected with the proper combination of color sensing pixels.
In a conventional complementary metal oxide semiconductor (CMOS) imager, each pixel cell in an array of pixels operates to convert light intensity to electrical charge, accumulate the electrical charge in proportion to the light intensity, and transfer the accumulated charge to an amplifier. In many CMOS imagers, a pixel may be reset to a specific reference voltage level prior to, or after, acquiring the image. This reference level may be used to compare a voltage level read from the pixel after exposure to light relative to the reference voltage level. With this configuration, a differential amplifier, or comparator, may be used to determine the difference between the exposed voltage level of a pixel and the reference voltage level of the pixel.
However, amplifiers used to amplify this difference generally include an offset voltage due to imbalances of the transistors, resistors, and other internal elements of the amplifier. This offset is difficult to predict and may change over process, temperature, and voltage variations. Furthermore, the offset may be amplified along with the intended signal, creating an even larger and unknown amount of offset on the amplified output signal.
Consequently, circuits and methods have been proposed for canceling this offset voltage so that the amplified output signal includes only amplification of the input signal and not the offset voltage. However, in image sensors, a single amplifier may be used for a large number of pixel columns to amplify the value from each pixel column in sequence. This large array of pixel columns may impose a large load on the input terminals of an amplifier. This large load may cause difficulties in using conventional offset cancellation techniques.
Therefore, there is a need for devices and methods for amplifier-offset compensation that can operate effectively with potentially large loads on the input signals, which may be attributable to an array of pixel columns coupled in parallel.